TGF-β
Transforming growth factor-β (TGF-β) signaling is implicated in a numerous diseases and conditions, including stroke, heart disease, bone loss, cancer, multiple sclerosis, wound healing, inflammation, and neurodegenerative disorders. TGF-β is a member of a superfamily of conserved cytokines, growth factors, and morphogens, which play key functions in development and homeostasis.9-11 The TGF-β subfamily includes three isoforms in mammals, TGF-β1, 2 and 3, which promote cell survival, induce apoptosis, stimulate cell proliferation, induce differentiation, and/or initiate or resolve inflammation, depending on the particular cell type and environment. Accurate regulation of TGF-β bioactivity and signaling is key to controlling these functions and essential to health and normal aging. The disruption of TGF-β signaling molecules frequently results in embryonic lethality in mice.12,13 
The biological actions of TGF-βs are mediated by a receptor complex consisting of the TGF-β type 1 (TBR1/ALK5) and type 2 (TBR2) serine/threonine kinase receptor subunits.9,10 Receptor activation leads to phosphorylation of Smad proteins, which translocate to the nucleus where they bind to the Smad DNA-binding element (SBE) present in an estimated 400 genes.14 TGF-βs can also activate other signaling pathways including the p38 MAP kinase pathway and the JNK or NF-kB pathways.11 Despite interaction with other pathways, knockout studies in mice suggest that Smad proteins are the key mediators of many of TGF-β1's actions in vivo.13 
TGF-B in the CNS
TGF-β is known to play a role in neurological diseases and condition. In the normal central nervous system (CNS), TGF-β1, 2, and 3, and their receptors are expressed in neurons, astrocytes, and microglia.4,5 The best studied isoform, TGF-β1, is expressed in the adult CNS predominantly in response to CNS injury, and may function as an organizer of protective and regenerative responses.15 It is upregulated in glial cells in response to brain lesioning, transient forebrain ischemia, and stroke.5 TGF-β2 and TGF-β3 bind to the same receptors as TGF-β1 but have different patterns of activation and expression.10,16 Immunoreactivity to TGF-β2 and TGF-β3 is detected in astrocytes and neurons in the normal CNS and is increased in neurodegenerative diseases or after stroke.4,5 Changes in TGF-β expression are reported in AD brain, cerebrospinal fluid (CSF), and serum.3,17-22 TGF-β1 immunoreactivity is increased in (or near) amyloid plaques19,20 and around cerebral blood vessels.3,18,22 
TGF-β Protection of Neurons
TGF-β1 has been demonstrated to protect neurons against various toxins and injurious agents in cell culture and in vivo.4,5,32 Astroglial overexpression of TGF-β1 in transgenic mice protected against neurodegeneration induced with the acute neurotoxin kainic acid or associated with chronic lack of apolipoprotein E expression.7 Boche and coworkers also demonstrated that TGF-β1 protects neurons from excitotoxic death.36 
Several mechanisms have been postulated to explain how TGF-β1 protects neurons. For example, TGF-β1 decreases Bad, a pro-apoptotic member of the Bcl-2 family, and contributes to the phosphorylation, and thus inactivation, of Bad by activation of the Erk/MAP kinase pathway.37 On the other hand, TGF-β1 increases production of the anti-apoptotic protein Bcl-2.38 TGF-β1 has also been shown to synergize with neurotrophins and/or be necessary for at least some of the effects of a number of important growth factors for neurons, including neurotrophins, fibroblast growth factor-2, and glial cell-line derived neurotrophic factor.32,39 In addition, TGF-β1 increases laminin expression40 and is necessary for normal laminin protein levels in the brain.7 It is also possible that TGF-β1 decreases inflammation in the infarction area, attenuating secondary neuronal damage.35 
Transgenic Animals
TGF-β1 transgenic mice overexpressing a secreted, constitutively active form of TGF-β1 in astrocytes at modest levels develop age-related cerebrovascular abnormalities including thickening of the capillary basement membrane and cerebrovascular amyloid deposition,22,29 nevertheless, these mice have better cognitive function than nontransgenic controls. Similar microvascular abnormalities are typical for AD and consistent with the observation that TGF-β1 mRNA levels in brains of AD cases correlate positively with vascular amyloid deposition.22 
TGF-β1 transgenic mice cross-bred with human amyloid precursor (hAPP) transgenic mice, develop synaptic degeneration and amyloid plaques in the brain parenchyma. Unexpectedly, a prominent reduction in plaque formation and overall Aβ accumulation was found in hAPP/TGF-β1 double transgenic compared with hAPP mice.3 Most of the remaining amyloid accumulated around cerebral blood vessels.
Increased levels of TGF-β1 reduced the number of plaques in human amyloid precursor protein (hAPP) mice by 75% and overall Aβ levels by 60%, compared to mice with normal levels of TGF-β1. Interestingly, TGF-β1 stimulated microglial cells to degrade synthetic Aβ peptide in culture. Because TGF-β1 also caused an activation of microglia in hAPP/TGF-β1 mice, these data suggest that at least some of the effects of TGF-β1 involve the activation of microglial phagocytosis.
The need exists for more effective pharmaceutical compounds for treating and preventing stroke, heart disease, bone loss, cancer, multiple sclerosis, wound healing, inflammation, and neurodegenerative disorders. The present compositions and methods involve small-molecules that modulate TGF-β signaling.